The present invention relates to a method and system for implementing a virtual line-switched ring network; such as a line switched ring network carrying optical signals in accordance with a synchronous optical network (SONET) standard.
SONET networks often have a ring configuration including a collection of nodes forming a closed loop. FIG. 1 illustrates an example of a conventional SONET bidirectional ring 100 whereby information may flow in either a clockwise or counterclockwise in the figure, as indicated by arrows labeled “working” and “protect”. Add-drop multiplexers (A/D mux) 110, 120, 130 and 140 add and/or drop signals to switch data from one span (SP1 to SP7) to another. Ring 100 is thus termed a “bidirectional line switched ring” or BLSR, and data transmitted in such a ring typically must conform to a particular protocol.
As further shown in FIG. 1, each of spans SP1 to SP7 includes one working line and a corresponding protection line. For example, spans SP1 and SP5 interconnect A/D muxes 110 and 120 and include working lines carrying data in opposite directions. The working lines within each of these spans further include respective protection lines for transmitting data in the event the associated working line fails.
The SONET ring provides protection for transmission of data in two way. First if a working lines fails, the corresponding protection lines may be used. In the alternative, if working lines fail between two A/D muxes, any communication route directed through the failed line may be rerouted through the A/D muxes through a process known as span switching. For example, if the working lines between A/D mux 110 and A/D mux 120 fail, instead of using the corresponding protection lines, communications may be sent from A/D mux 110 to A/D mux 120 via A/D mux 140 and 130.
Typically, the working and protect lines are provided in a fiber optic bundle. Accordingly, if the working line fails, due to a fiber cut, for example, the corresponding protect line often will also fail. Span switching is thus often preferred to simply switching data from the faulty working line to the protect line. Both schemes may be used in conjunction with each other, however, whereby an attempt is first made to use the protect line when the associated working line fails, and then, if the protection line is itself faulty, span switching is used to redirect communications.
The SONET standard has a plurality of optical levels and logical levels that represent the amount of optical information a line is capable of carrying at a given time. These different optical levels are referred to as OC-n, where n is indicative of the bandwidth or capacity associated with the line. Current SONET bidirectional rings require that all spans carry data at the same optical rate because A/D muxes can only direct communications from one line to another having the same OC-n level. Therefore, BLSR requires that all lines in the network are of the same type and that each span between A/D muxes has the same number of lines.
In accordance with the SONET standard, spans transfer units of information called Synchronous Transport Signals (STS). For the different optical carrier levels OC-n (such as OC-1, OC-3 and OC-12), there is a corresponding STS-n, where n is the number of STS-1 segments or time slots. Typical spans are composed of 1, 3, 12, 48, or 192 STS-1's. All SONET spans transmit 8,000 frames per second, where each frame is composed of an integer number of STS-1 segments, such as 1, 3, 12, 48 or 192.
Each STS-1 segment includes a payload section and an overhead section. The overhead includes K-bytes that communicate error conditions between spans in a network and allow for link recovery after network failure. K-byte signaling takes place over the protection lines. In a series of STS segments, only K-bytes from the first STS-1 segment are used to carry error data. Current SONET networks make no use of the framing overhead of the remaining STS-1 segments. The series of STS-1 segments only carries K-byte error information for a single ring.
FIG. 2 illustrates an example of a connection between two rings 200 and 210 using four SONET A/D multiplexors. Specifically, A/D mux 202 of ring 200 is coupled to A/D mux 212 of ring 210, while A/D mux 206 of ring 200 is coupled to A/D mux 216 of ring 210. Data is transmitted on these connections at a slower rate than through rings 200 and 210. Thus, a total of four “matched” A/D mux nodes are often required to connect two rings. Typically, each such pair of A/D muxes is dedicated to providing ring-to-ring connections, and are not configured to pass information around a ring and forward information to another ring at the same time.
In the SONET network ring environment, there currently does not exist a ring configuration that allows for spans within a single ring to have different bandwidth or for a different number of lines to exist between nodes. In addition, no current SONET network ring allows for sharing protection lines between different rings. Finally, current SONET network rings do not allow for connecting rings using a single node.